1. Field of the Invention
The present invention relates to multiple static pressure measuring systems using one external, strut mounted probe.
2. Description of the Prior Art
Various types of compensated dual static tubes have been utilized wherein dual pressure sensing ports are used so that redundancy of compensated static pressure from the remote instrument is established. U.S. Pat. No. 3,482,445 describes a strut mounted dual static tube. It is designed to provide two pressure outputs of equal level magnitude. A tapered transition surface section is used to provide the desired change in pressure along the tube.
Typically, one static line is used for operating the pilot's primary instruments such as an airspeed indicator, an altimeter, a rate of climb indicator and a Mach meter. The same instruments in the co-pilot's secondary system are connected to the second static line. Air data computers, autopilots, flight recorders and the like are sometimes also connected into the pilot's and/or co-pilot's static pressure systems or they could require a third or fourth static system on the aircraft. Some aircraft require up to six static tubes to provide the required pressure outputs with the necessary redundancy.
In any probe the position of the static pressure port on the probe has a bearing upon the measured pressure relative to actual pressure. Any error due to pressure disturbances near the aircraft must be compensated for either by the tube itself, or in the instruments or a computer, to relate the measured pressure to the true static pressure. The compensation problems are multipled when a strut is used for mounting the probe because of the airflow and pressure patterns around the strut itself, which disrupt the normal measured static pressure. The pressure pattern around the strut varies greatly with different speeds of the aircraft and different atmospheric conditions.
If the probes could be greatly elongated, the problems would be simplified because the pressure pattern would stabilize. However, the lengths necessary to get a stable pressure pattern so that two spaced apart static pressure ports will sense the same measured static pressure is greater than that desirable or permissible for probes which are mounted on the fuselage or aircraft.
The requirement thus is for a short strut-mounted probe which has multiple sets of static pressure sensing ports and wherein each set of ports measures the same pressure. An additional requirement is that the ports measure a pressure which has a known and selectable relation to the local static pressure at the probe location. For some applications, especially on helicopters and small commuter aircraft, there is a need for much higher pressure levels, both positive and negative, than can be obtained from the design described in U.S. Pat. No. 3,482,445.